1. Field of the Invention
This invention relates to improved jamming suppression techniques and, more specifically, to a technique for improved jamming suppression of spread spectrum antenna/receiver systems, such as the Global Positioning System (GPS).
2. Brief Description of the Prior Art
Spread spectrum communication is well known and is a means of communicating by purposely spreading the spectrum (frequency extent or bandwidth) of the communication signal well beyond the bandwidth of the unspread signal. Spread spectrum signals are typically transmitted by electromagnetic waves in free space with usage in both non-military and military systems.
Spread spectrum communication and/or navigation receivers have some natural jamming immunity that is inherent to the way that matched filter receivers operate. However, there is a limit to this natural jamming rejection capability. When a jammer (either inadvertently friendly or hostile) produces an amount of jamming power at the antenna of the spread spectrum receiver that exceeds the limit of the receiver, the receiver is then unable to recover the desired information. For example, in the case of a GPS receiver, a 0 dBi antenna receives a signal from the GPS satellites at a level of approximately -133 dBm. There is some spread spectrum processing gain against jamming signals, however, initial acquisition becomes difficult when the jamming signal is approximately 30 dB stronger than the GPS signal, such jamming signal then being only -133 dBm+30 dBm=-103 dBm. It is therefore not particularly difficult for a hostile or inadvertent friendly jammer to exceed this amount of jamming power in the GPS receiver/antenna system. When this jamming situation occurs, a standard GPS receiver cannot acquire the GPS signals. Since the receiver cannot track GPS signals, the result is an inability to develop a navigation solution based upon GPS. Though the discussion herein will generally be provided using GPS as a principal example, it should be understood that the techniques described are applicable to any spread spectrum communications or navigation system which is required to acquire and track radio signals arriving simultaneously from multiple angles and that is being jammed by electromagnetic radiations.
Spread spectrum communication/navigation system receivers, such as GPS, typically operate under conditions where the signal power is substantially lower than the total receiver front end thermal noise power in the spread spectrum bandwidth of the signal. In the specific case of GPS, the GPS signal can easily be 30 dB to 40 dB below the receiver front end noise power level in the 10.23 Mhz chip rate bandwidth. The matched filter processing improves the signal to noise ratio (SNR) by the time-bandwidth product which is 53 dB for GPS P(Y) code and 43 dB for C/A code. The result of this matched filter processing is to increase the net SNR well above 0 dB so that accurate parameter estimates can be made. The fact that the spread spectrum signal power is well below the receiver front end noise power level at the point in the signal processing flow where both the temporal narrowband jammer rejection and spatial adaptation wideband jammer rejection are accomplished is very significant. In many adaptive cancellation applications where the desired signal is at or above the receiver front end noise level, the adaptive algorithm has a tendency to cancel the desired signal. In GPS, for example, the desired signal is so far below the front end noise level of the receiver that the jammer cancellation algorithms cannot sense the desired signal sufficiently well to cancel it through adaptation. Consequently, the jammer suppression algorithms may freely adapt to the frequencies of the jammers with only one exception. This exception is when there are more narrowband jammers than the degrees of freedom provided in the temporal nulling assets can effectively null, in which case some of the narrowband jamming signals will reach the spatial adaptation process where they will be nulled if there are sufficient remaining degrees of freedom, considering the number of broadband jammers present. The spatial adaptation algorithm should spend one of its degrees of freedom protecting the single angle of its designated GPS satellite signal.
In general, signal jammers are of two types, one type having a very narrow bandwidth, such as, for example, a sine wave. Such jammers are suppressed by operating in the time domain with notch filters which filter out the jamming frequency or frequencies from the filter output while passing the remainder of the received signal. In this way, only a small part of the GPS signal is removed with a sufficient amount of the GPS signal remaining to permit appropriate operation therewith. The other type of jammer is broadband and generally has a bandwidth substantially the same as the GPS signal. In this case, a notch filter which filters out the jamming frequencies will also filter out most of or all of the GPS signal, with an insufficient portion of the GPS signal remaining to permit appropriate operation therewith. Accordingly, jamming suppression of jamming signals having substantially the same bandwidth as the GPS signal is instead provided by a procedure known as spatial filtering. With spatial filtering, the direction from which the signal is received is taken into account with the filtering action being based upon such direction rather than frequency. Therefore, as long as the jamming signal is not travelling to the receiver in the same direction as the GPS signal, the jamming signal can be filtered out by such well known spatial filtering techniques.
While the above described prior art jamming suppression techniques have been successfully employed, there is always a need for even better jamming suppression techniques to combat improvements in jamming techniques.